Retinal Projections in the Lemon Shark (Negaprion brevirostris)

Abstract

Modifications of the Nauta method were used to reveal the optic pathways of Negaprion brevirostris following unilateral enucleation. Postoperative survival times ranged from 7 to 49 days with no degeneration apparent before 21 days. Completely crossed projections to the optic tectum as well as to four distinct thalamic cell groups are described. While there was evidence of a bilateral projection to the hypothalamus, there was none indicating the existence of a posterior accessory optic tract. Terminal degeneration in the optic tectum appeared relatively homogenous and was limited to the stratum griseum et fibrosum superficiale. The results confirm previous observations in two other species of sharks and suggest the importance of nontectal structures in shark vision.